An electronic device, such as a liquid crystal display device, is provided with a protection circuit that protects the electronic device from an overvoltage caused by, for example, static electricity. The protection circuit is provided between a signal line connected to, for example, the electronic device and a ground line, and allows current to flow to the ground line when an overvoltage is applied to the signal line, thereby protecting the electronic device.
As the protection circuit, the circuit has been developed which has a structure in which a source electrode and a drain electrode of two thin film transistors (TFT) are connected in parallel or in series to each other in an opposite direction.
However, when two oxide semiconductor TFTs of thin film transistors are connected in parallel to each other to form the protection circuit, overcurrent may flow even if a voltage within a driving voltage range of an electronic device is applied, because the threshold voltage and sub-threshold swing (SS) of the oxide semiconductor TFT are less than those of an amorphous silicon TFT (a-Si TFT). When two oxide semiconductor TFTs are connected in series with each other to form the protection circuit, there is a case where no current flows to the earth line even if an overvoltage is applied to the signal line because off current is very small.
For example, Japanese Patent Application Laid-Open No. S63-220289 (hereinafter, referred to as Patent Document 1) discloses the protection circuit which connects a shunt wire and a wire to be protected from static electricity using two TFTs in which a drain or a source and a gate are connected to each other. However, in the protection circuit disclosed in Patent Document 1, in a case in which an oxide semiconductor TFT is used as the TFT, even if a voltage for normally operating the liquid crystal display device is applied, a large amount of current is possibly to flow to the shunt wire through the protection circuit. This is caused by the characteristics of the oxide semiconductor TFT. In the characteristics, a threshold voltage is low and a sub-threshold swing is small.
Japanese Patent Application Laid-Open No. H08-146460 (hereinafter, referred to as Patent Document 2) discloses the discharge circuit in which a plurality of protection circuits are connected in series to each other to control an operating voltage. In the discharge circuit disclosed in Patent Document 2, when the operating voltages of the protection circuits are equal to each other, n protection circuits are connected in series to each other to increase the operating voltage by n times.
Japanese Patent Application Laid-Open No. 2009-187029 (hereinafter, referred to as Patent Document 3) discloses the protection circuit in which a voltage that is generated due to static electricity is divided by two resistors and the divided voltages are applied to a gate of transistors forming the protection circuit to control an operating voltage.
Japanese Patent Application Laid-Open No. 2005-136028 (hereinafter, referred to as Patent Document 4) discloses an electrostatic protection circuit in which a double-gate (dual-gate) thin film transistor is used, a top gate electrode of the thin film transistor is connected to a source electrode and one connection terminal, a bottom gate electrode of the thin film transistor is connected to a drain electrode and the other connection terminal. The ON/OFF states of the double-gate thin film transistor are controlled on the basis of a gate voltage which is applied to the top gate electrode and the bottom gate electrode.
In the discharge circuit disclosed in Patent Document 2, when the operating voltage of each protection circuit is low, the number of protection circuits which are required to be operated at a desired voltage increases, which results in an increase in the size of the discharge circuit. In addition, the amount of current during the operation of the discharge circuit is reduced in inverse proportion to the number of protection circuits. Therefore, when the number of protection circuits increases, the capability of the discharge circuit to make static electricity flow is reduced.
In the protection circuit disclosed in Patent Document 3, when the resistance value of the resistor used for voltage division is not appropriate, the amount of current flowing through the resistor increases. Therefore, a material suitable for forming the resistor is limited. As a result, in some cases, an appropriate resistor is not obtained and it is difficult to adjust the gate voltage of the transistor provided in the protection circuit to a desired voltage.
The electrostatic protection circuit disclosed in Patent Document 4 uses the difference between the threshold voltage of the bottom gate and the threshold voltage of the top gate. Patent Document 4 does not disclose a method for arbitrarily controlling the threshold voltages of the bottom gate and the top gate.